Our objective is to investigate the role of the aryl hydrocarbon receptor (AhR) in breast cancer progression with a long-term goal of targeting AhR for preventive and therapeutic intervention of breast cancer. AhR is a ligand- activated transcription factor which mediates the carcinogenic effects of environmental polycyclic aromatic hydrocarbons (PAH). PAH activation of AhR is implicated in carcinogenesis through induction of CYP1 family of genes with subsequent generation of reactive metabolites and activation of transcription factors. Our laboratory and others have reported high-level expression of constitutively activated AhR in advanced human breast carcinomas. We further showed that ectopic over expression of AhR is sufficient to induce malignant transformation of human mammary epithelial cells (HMEC), independent of PAH. Our generated clonal cell lines of the AhR-transformed HMEC exhibited enhanced proliferation, epithelial-to-mesenchymal transition and enhanced migration and invasiveness, as measured in vitro. We therefore postulated that elevated expression of AhR correlates with and forecasts breast cancer progression and that high AhR expression promotes the development of invasive breast carcinoma by up-regulating signaling pathways critical for tumor survival and invasiveness. To address this hypothesis our specific aims are 1) To test the relevance of AhR over expression for development and progression of invasive carcinoma in vivo using SCID mice xenograft model, by orthotopically implanting clonal HMEC cell lines which over express AhR and demonstrating their ability to form mammary tumors and metastatic occults 2) To examine the causal role of the AhR over expression on the development and progression of invasive carcinoma by the siRNA knockdown of AhR expression in cells overexpressing AhR followed by assessing their tumorigenic potential in vitro and in SCID mice 3) To determine which genes and signaling pathways for tumor progression are activated by AhR over expression; 4) To examine whether the expression of AhR is universally upregulated in advanced human breast cancer, by correlating the AhR expression in human breast tumors with the stage of the disease and other prognostic biomarkers using tissue microarrays. The proposed research will define the causal role of AhR in invasive breast cancer progression. Our findings may validate the AhR as a new predictive clinical marker and a unique target for designing novel selective inhibitors for therapeutic intervention of metastatic breast cancer.